This invention relates generally to medical device therapy for diabetes, more specifically to a medical device system for adjunct therapy of diabetes by neuromodulation of a selected nerve or nerve bundle utilizing an implanted lead-receiver and an external stimulator.
Diabetes is a significant health problem affecting millions of Americans. It is the leading risk factor in coronary heart disease and stroke, leading cause of blindness, end-stage renal disease, and a major contributor to lower extremity amputations. Diabetes mellitus is a heterogeneous group of diseases, all of which ultimately lead to hyperglycemia (an elevation of glucose in the blood) and excretion of glucose in the urine as hyperglycemia increases. Diabetes mellitus is also characterized by the inability to reabsorb water, which results in increased urine production (polyuria), excessive thirst (polydipsia), and excessive eating (polyphagia).
The regulation of glucose levels in the body is by the hormone insulin. Insulin is produced by the beta cells of the pancreas from the tissue called islets of Langerhans (shown schematically in FIG. 1). The pancreas is a flattened organ located just below the stomach. Insulin lowers blood sugar concentration by facilitating the movement of glucose into body tissues. Insulin is intimately involved in the regulation not only of glucose metabolism but also of protein and fat metabolism. Therefore, it is not suprising that all of the major foodstuffs play some role in regulating insulin release. Even though blood glucose level is the most important determinant that increases insulin secretion, other factors and conditions also influence insulin release. Among the other factors, and most notably for this invention is the condition that parasympathetic stimulation also increases insulin secretion.
Nerves have trophic influences on tissues, and the secretion of insulin is stimulated by vagal nerve fibers. Decreased glucose tolerance following vagotomy (interruption of the imulses carried by the vagus nerve) has been reported in human subjects, and sympathetic stimulation via the splanchnic nerve inhibits insulin release. The central nervous system also plays a role in regulating insulin secretion, and the outflow probably occurs via the hypothalamus and the autonomic nervous system.
In this invention diabetes is treated by electrical stimulation neuromodulation of the vagal nerve fibers with an implanted lead-receiver and an external stimulator with predetermined programs. The neuromodulation thus increasing the secretion of insulin by the pancreas, for adjunct (add-on) treatment of diabetes.
Observations on the profound effect of electrical stimulation of the vagus nerve on central nervous system (CNS) activity extends back to the 1930""s. The vagus nerve 54 provides an easily accessible, peripheral route to modulate central nervous system (CNS) function. Other cranial nerves can be used for the same purpose, but the vagus nerve 54 is preferred because of its easy accessibility. In the human body there are two vagus nerves (VN), the right VN and the left VN. Each vagus nerve is encased in the carotid sheath along with the carotid artery and jugular vein. The innervation of the right and left vagus nerves is different. The innervation of the right vagus nerve is such that stimulating it results in profound bradycardia (slowing of the heart rate). The left vagus nerve has some innervation to the heart, but mostly innervates the visceral organs such as the gastrointestinal tract. It is known that stimulation of the left vagus nerve does not cause any significant deleterious side effects.
One of the fundamental features of the nervous system is its ability to generate and conduct electrical impulses. These can take the form of action potentials, which is defined as a single electrical impulse passing down an axon, and is shown schematically in FIG. 2. The top portion of the figure shows conduction over mylinated axon (fiber) and the bottom portion shows conduction over nonmylinated axon (fiber). These electrical signals will travel along the nerve fibers.
The nerve impulse (or action potential) is an xe2x80x9call or nothingxe2x80x9d phenomenon. That is to say, once the threshold stimulus intensity is reached an action potential 7 will be generated. This is shown schematically in FIG. 3. The bottom portion of the figure shows a train of action potentials.
Most nerves in the human body are composed of thousands of fibers of different sizes. This is shown schematically in FIG. 4. The different sizes of nerve fibers, which carry signals to and from the brain, are designated by groups A, B, and C. The vagus nerve, for example, may have approximately 100,000 fibers of the three different types, each carrying signals. Each axon or fiber of that nerve conducts only in one direction, in normal circumstances.
In a cross section of peripheral nerve it is seen that the diameter of individual fibers vary substantially. The largest nerve fibers are approximately 20 xcexcm in diameter and are heavily myelinated (i.e., have a myelin sheath, constituting a substance largely composed of fat), whereas the smallest nerve fibers are less than 1 xcexcm in diameter and are unmyelinated. As shown in FIG. 5, when the distal part of a nerve is electrically stimulated, a compound action potential is recorded by an electrode located more proximally. A compound action potential contains several peaks or waves of activity that represent the summated response of multiple fibers having similar conduction velocities. The waves in a compound action potential represent different types of nerve fibers that are classified into corresponding functional categories as shown in the table below,
The diameters of group A and group B fibers include the thickness of the myelin sheaths. Group A is further subdivided into alpha, beta, gamma, and delta fibers in decreasing order of size. There is some overlapping of the diameters of the A, B, and C groups because physiological properties, especially in the form of the action potential, are taken into consideration when defining the groups. The smallest fibers (group C) are unmyelinated and have the slowest conduction rate, whereas the myelinated fibers of group B and group A exhibit rates of conduction that progressively increase with diameter.
Compared to unmyelinated fibers, myelinated fibers are typically larger, conduct faster, have very low stimulation thresholds, and exhibit a particular strength-duration curve or respond to a specific pulse width versus amplitude for stimulation. The A and B fibers can be stimulated with relatively narrow pulse widths, from 50 to 200 microseconds (xcexcs), for example. The A fiber conducts slightly faster than the B fiber and has a slightly lower threshold. The C fibers are very small, conduct electrical signals very slowly, and have high stimulation thresholds typically requiring a wider pulse width (300-1,000 xcexcs) and a higher amplitude for activation. Because of their very slow conduction, C fibers would not be highly responsive to rapid stimulation. Selective stimulation of only A and B fibers is readily accomplished. The requirement of a larger and wider pulse to stimulate the C fibers, however, makes selective stimulation of only C fibers, to the exclusion of the A and B fibers, virtually unachievable inasmuch as the large signal will tend to activate the A and B fibers to some extent as well.
The vagus nerve is composed of somatic and visceral afferents and efferents. Usually, nerve stimulation activates signals in both directions (bi-directionally). It is possible however, through the use of special electrodes and waveforms, to selectively stimulate a nerve in one direction only (unidirectionally). The vast majority of vagus nerve fibers are C fibers, and a majority are visceral afferents having cell bodies lying in masses or ganglia in the skull. The central projections terminate largely in the nucleus of the solitary tract, which sends fibers to various regions of the brain (e.g., the thalamus, hypothalamus and amygdala).
Vagus nerve stimulation is a means of directly affecting central function. As shown in FIG. 6, cranial nerves have both afferent pathway 19 (inward conducting nerve fibers which convey impulses toward the brain) and efferent pathway 21 (outward conducting nerve fibers which convey impulses to an effector). The vagus nerve 54 is composed of 80% afferent sensory fibers carrying information to the brain from the head, neck, thorax, and abdomen. The sensory afferent cell bodies of the vagus reside in the nodose ganglion and relay information to the nucleus tractus solitarius (NTS).
FIG. 7 shows the nerve fibers traveling through the spinothalamic tract to the brain. The afferent fibers project primarily to the nucleus of the solitary tract (shown schematically in FIG. 8) which extends throughout the length of the medulla oblangata. A small number of fibers pass directly to the spinal trigeminal nucleus and the reticular formation. As shown in FIG. 8, the nucleus of the solitary tract has widespread projection to cerebral cortex, basal forebrain, thalamus, hypothalamus, amygdala, hippocampus, dorsal raphe, and cerebellum. In summary neuromodulation of the vagal nerve fibers increase the secretion of insulin by the pancreas, because of the connections of the nucleus tractus solitarus to the appropriate centers in the brain.
One type of non-pharmacologic therapy for diabetes is generally directed to the use of an implantable lead and an implantable pulse generator technology or xe2x80x9ccardiac pacemaker-likexe2x80x9d technology, i.e. stimulation with an implantable Neurocybernetic Prosthesis. In the prior art, the pulse generator is programmed via a xe2x80x9cpersonnel computer (PC)xe2x80x9d based programmer that is adapted with a programmer wand, which is placed on top of the skin over the pulse generator implant site. This is shown in FIG. 9. Also, in the prior art each parameter is programmed independent of the other parameters. Therefore, millions of different combinations of programs are possible. In the current patent application, limited number of programs, say less than 20, are built-in. Some of the other prior art is summarized below.
U.S. Pat. No. 3,796,221 (Hagfors) is directed to controlling the amplitude, duration and frequency of electrical stimulation applied from an externally located transmitter to an implanted receiver by inductively coupling. Electrical circuitry is schematically illustrated for compensating for the variability in the amplitude of the electrical signal available to the receiver because of the shifting of the relative positions of the transmitter-receiver pair. By highlighting the difficulty of delivering consistent pulses, this patent points away from applications such as the current application, where consistent therapy needs to be continuously sustained over a prolonged period of time. The methodology disclosed is focused on circuitry within the receiver, which would not be sufficient when the transmitting coil and receiving coil assume significantly different orientation, which is likely in the current application. The present invention discloses a novel approach for solving this problem.
U.S. Pat. Nos. 4,702,254, 4,867,164 and 5,025,807 (Zabara) generally disclose animal research and experimentation related to epilepsy and the like and are directed to stimulating the vagas nerve by using xe2x80x9cpacemaker-likexe2x80x9d technology, such as an implantable pulse generator. The pacemaker technology concept consists of a stimulating lead connected to a pulse generator (containing the circuity and DC power source) implanted subcutaneously or submuscularly, somewhere in the pectoral or axillary region, and programming with an external personal computer (PC) based programmer. Once the pulse generator is programmed for the patient, the fully functional circuitry and power source are fully implanted within the patient""s body. In such a system, when the battery is depleted, a surgical procedure is required to disconnect and replace the entire pulse generator (circuitry and power source). These patents neither anticipate practical problems of an inductively coupled system, nor suggest solutions to the same for an inductively coupled system for neuromodulation therapy.
U.S. Pat. No. 5,231,988 (Wernicke et al) is generally directed to treatment of endocrine disorders by nerve stimulation with an implantable neurocybernetic prosthesis (NCP).
U.S. Pat. No. 5,304,206 (Baker, Jr. et al) is directed to activation techniques for implanted medical stimulators. The system uses either a magnet to activate the reed switch in the device, or tapping which acts through the piezoelectric sensor mounted on the case of the implanted device, or a combination of magnet use and tapping sequence.
U.S. Pat. No. 4,573,481 (Bullara) is directed to an implantable helical electrode assembly configured to fit around a nerve. The individual flexible ribbon electrodes are each partially embedded in a portion of the peripheral surface of a helically formed dielectric support matrix.
U.S. Pat. No. 3,760,812 (Timm et al.) discloses nerve stimulation electrodes that include a pair of parallel spaced apart helically wound conductors maintained in this configuration.
U.S. Pat. No. 4,979,511 (Terry) discloses a flexible, helical electrode structure with an improved connector for attaching the lead wires to the nerve bundle to minimize damage.
Apparatus and method for neuromodulation, in the current application has several advantages over the prior art implantable pulse generator. The external stimulator described here can be manufactured at a fraction of the cost of an implantable pulse generator. The therapy can be freely applied without consideration of battery depletion. Surgical replacement of pulse generator is avoided. The programming is much simpler, and can be adjusted by the patient within certain limits for patient comfort. And, the implanted hardware is much smaller.
The system and method of the current invention also overcomes many of the disadvantages of the prior art by simplifying the implant and taking the programmability into the external stimulator. Further, the programmability of the external stimulator can be controlled remotely, via the wireless medium, as described in a co-pending application. The system and method of this invention uses the patient as his/her own feedback loop. Once the therapy is prescribed by the physician, the patient can receive the therapy as needed based on symptoms, and the patient can adjust the stimulation within prescribed limits for his/her own comfort.
The present invention is directed to system and methods for adjunct (add-on) electrical neuromodulation therapy for diabetes and blood glucose regulation, using predetermined programs with an external stimulator. The system consists of an implantable lead-receiver containing passive circuitry, electrodes, and a coil for coupling to the external stimulator. The external stimulator, which may be worn on a belt or carried in a pocket contains, electronic circuitry, power source, primary coil, and predetermined programs. The external primary coil and subcutaneous secondary coil are inductively coupled. The patient may selectively activate stimulation corresponding to meals, or leave the stimulation on according to predetermined program.
In one aspect of the invention the pulse generator contains a limited number of predetermined programs packaged into the stimulator, which can be accessed directly without a programmer. The limited number of programs can be any number of programs even as many as 100 programs, and such a number is considered within the scope of this invention.
In another feature of the invention, the system provides for proximity sensing means between the primary (external) and secondary (implanted) coils. Utilizing current technology, the physical size of the implantable lead-receiver has become relatively small. However, it is essential that the primary (external) and secondary (implanted) coils be positioned appropriately with respect to each other. The sensor technology incorporated in the present invention aids in the optimal placement of the external coil relative to a previously implanted subcutaneous coil. This is accomplished through a combination of external and implantable components.
In another feature of the invention, the external stimulator has predetermined fixed programs, as well as a manual xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d button. Each of these programs has a unique combination of pulse amplitude, pulse width, frequency of stimulation, on-time and off-time. After the therapy has been initiated by the physician, the patient has a certain amount of flexibility in adjusting the intensity of the therapy (level of stimulation). The patient has the flexibility to decrease (or increase) the level of stimulation (within limits). The manual xe2x80x9conxe2x80x9d button gives the patient flexibility to immediately start the stimulation pattern at any time. Of the pre-determined programs, patients do not have access to at least one of the programs, which can be activated only by the physician, or an appropriate medical person.